The invention relates to a clamping unit, in particular for an injection molding machine.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Clamping units, in particular for injection molding machines, typically include two platens for receiving molding tools, with at least one platen movably supported on a machine bed. This moving platen can travel back and forth for closing and opening the clamping unit and is typically supported on a frame element of a machine bed by a sliding bearing with hydrodynamic lubrication. Thus, a lubricant is introduced or applied between the bearing elements, and a lubricating film is formed during a sliding motion between the elements to be supported. The lubricant can be introduced, for example, via a lubrication groove. The friction coefficient in such a hydrodynamic lubrication or bearing support is speed-dependent, whereby friction decreases at relatively high speeds. The lower the speed, the greater is typically the increase in the friction due to a possible reduced thickness of the lubricating film (slip-stick effect, static friction effects). This effect is particularly disadvantageous, when safety-related issues must be ascertained at the end of a closing movement. Such safety-related issues may relate to a jamming of a not fully finished product between the two platens or a protrusion of a foreign object into the clamping unit. Such a problematic situation should be detected as quickly as possible and with a high sensitivity. In particular, when a slow end motion of the closing operation is involved, application of a greater force has been required heretofore as a result of the increase in friction, so that the sensitivity of the detection of malfunctions could not be sufficiently ensured.
It would therefore be desirable and advantageous to provide an improved clamping unit for an injection molding machine, to obviate prior art shortcomings and to attain a displacement of the movable platen with constant low friction also at low travel speeds of the moving platen.